1. Field of the Invention
The present invention relates to the field of charge transfer devices.
2. Prior Art
Many investigators have studied the effects of transfer inefficiency on the modulation transfer function in both signal processing and optical charge transfer devices. Transfer inefficiency in charge transfer devices exhibits itself as a frequency response degradation in signal processing devices, and as an image smearing in optical devices. In contrast, a few investigators have studied methods for compensating the effects of transfer inefficiency in the area of signal processing such as for a discrete time delay line, though no methods are known for compensation of charge transfer devices such as imaging devices having significant charge transfer inefficiencies. The lack of such information produces severe restrictions on the application of these devices, especially in areas of imaging where contrast ratios change monotonically and proportionally to the image position along the array axis, thus producing smearing from one end of the imaging device, though improving across the device to a well defined image at the other end.
The prior art methods for compensation in signal processing devices are applicable to circuits for signals which pass completely through the device, a condition not found in an imaging device. In particular, in an imaging device each pixel originates at a discrete position along the array. Consequently they do not all pass through every storage site in the charge transfer device, and hence leave behind varying amounts of signal residue that seemingly produce an indeterminate signal because of the fact that each signal pixel charge has had additions in different degrees proportional to the residue charge left behind by all preceeding pixels. Therefore the addition seemingly cannot be separated from the desired or observed pixel.
The principles which have been adopted and implemented with respect to signal delay line compensation have been discussed by many authors (Thornber, Karvel K., Optimum Linear Filtering for Charge-Transfer Devices, IEE Journal of Solid-State Circuits, vol. SC-9, No. 5, 1974; White, Marvin H., and Donald R. Lampe, Charge Coupled Devices (CCD) Analog Signal Processing, International Conference on the Application of Charge-Coupled Devices, Sponsored by Naval Lab Center, San Diego, CCD 75, 1975; and Sequin, Carlo H. and Michael F. Tompsett, Charge Transfer Devices, Academic Press, Inc., 1975. Probably the paper which has been referred to most is that of Karvel K. Thornber. In accordance with his work the effect of transfer inefficiency on the output of a signal processing device such as a delay device may be compensated for using a simple recursive filter on the output, the coefficients of which are selected dependent upon the transfer inefficiencies and the number of cells from input to output. However, for the reasons previously mentioned, such a compensation technique is not applicable to any devices wherein the total number of transfers required to shift any signal to the output varies, such as by way of example in imaging devices where the number of such transfers is dependent upon the relative position of the image segment.